The present invention relates to a hydraulic control apparatus for a continuous variable transmission mounted on a vehicle, and more particularly to a hydraulic control apparatus for controlling a line pressure supplied to a hydraulic cylinder of a driven pulley to prevent a belt member from slipping within a groove of the driven pulley.
A continuously variable transmission controls a speed ratio, also referred to as revolution ratio, between the driving and driven pulleys in such a manner that permits a minimum amount of fuel to be consumed.
The continuously variable transmission has a V-belt extending between a driving pulley and a driven pulley, the belt transferring the revolutions of the driving pulley to the driven pulley. The revolution ratio between the driven pulley and the driving pulley is controlled by varying the size of a V-shaped opening in the driving and driven pulleys within which the V-belt rotates. The size of the V-shaped opening is varied by changing the amount of pressure being fed into a hydraulic cylinder of the driving pulley and a hydraulic cylinder of the driven pulley. Hence, the effective diameter of the pulleys can be varied by changing the amount of force applied to the belt, thereby allowing for a continuously varying rotation ratio between the driven pulley and the driving pulley. The V-shaped openings on the driving and driven pulleys are each defined by an area between a fixed number of the pulley and a movable member of the pulley, the size of the V-shaped opening being dependent upon the pressure inputted to the respective hydraulic cylinders. In order to provide hydraulic pressure to each of the movable pulleys, a hydraulic control apparatus is provided.
The hydraulic control apparatus controls the line pressure supplied to the hydraulic cylinder of the driven pulley in an amount which is proportional to the amount of torque transmitted from the V-belt to the driven pulley. The hydraulic pressure in the hydraulic cylinder of the driving pulley controls the revolution ratio between the driven pulley and the driving pulley. The hydraulic control apparatus has a regulator valve which controls the amount of line pressure supplied to the hydraulic cylinder of the driven pulley. If the value of the supplied line pressure is less than a requisite amount for transmitting torque from the V-belt to the driven pulley, slipping occurs between the V-belt and the driven pulley making the tansmission of torque from the V-belt to the driven pulley inefficient. Conversely, if the amount of line pressure supplied to the hydraulic cylinder of the driven pulley is much greater than the requisite amount of line pressure for transmitting torque from the V-belt to the driven pulley, the result is severe wear upon the continuously variable transmission and the hydraulic fluid pump.
It is theoretically possible to maintain an optimum control of the line pressure supplied to the hydraulic cylinder of the driving pulley if the coefficient of friction generated between the V-belt and the driven pulley can be quantified. The coefficient of friction varies as a function of the temperature of the hydraulic fluid, the amount of wear exhibited by the V-belt and the rotating speed of the belt. Hence, a hydraulic control apparatus for a continuously variable transmission is designed to supply a line pressure greater than the requisite pressure to eliminate slippage between the V-belt and the driven pulley during all possible driving conditions and to ensure the transfer of torque between the V-belt and the driven pulley.
When the value of the line pressure gradually drops and becomes less than a predetermined value, the V-belt begins to slip on the surfaces of the driven pulley. When the V-belt begins to slip, a ratio between torques of the output shaft and input shaft rapidly decreases the amplitude of a vibration in torque of the driven pulley generated by a power stroke of an engine to the amplitude of a vibration of a torque of a crank shaft. In view of the above fact, it is possible to lower the value of the line pressure by detecting a variation in the torque ratio.